This invention relates to an apparatus for cooling and granulating plastic strands which emerge from nozzles in molten form and are fed, either directly or via a chute with a cooling water pipe to be disposed below the nozzles, to a following discharge channel having at least an apparatus for producing a coolant stream, the strands passing after the discharge channel into a granulator having at least a knife assembly consisting of a rotatable shearing knife and counterknife for granulating the strands.
DE 39 00 250 C2 discloses an apparatus for cooling, drying and granulating strands emerging from nozzles in molten form having a discharge channel disposed with the receiving-side end thereof under the nozzles, a device furthermore being provided for producing a coolant stream on the discharge channel, the discharge channel being followed by a granulator, and a drainage path in which the discharge channel is provided with openings for free passage of the coolant also being disposed in the discharge channel before the discharge-side end.
In this known apparatus the discharge channel is furthermore provided in the bottom thereof in the area after the drainage path with inlet nozzles for an air stream over such a length and so close together that the strands are guided along the discharge channel largely frictionlessly with respect to the bottom with a degree of dryness permitting immediate processing into the granulator disposed at the discharge-side end thereof. This granulator has within a suitable housing substantially two draw-in rolls one above the other for the plastic strands, and a cutter block cooperating with a corresponding counterknife. This cutter block and the counterknife serve to granulate the plastic strands fed by the draw-in rolls, and the granules consequently produced by the granulator fall out through a discharge shaft for further processing.
Depending on the application of such apparatuses it is sooner or later necessary to replace the wearing parts, in particular the cutting tools, which means that the apparatus must be shut down for the suitable maintenance and repair measures to be performed.
As mentioned above, this includes in particular replacing the components of the complete knife assembly, in particular the shearing knife and counterknife, but possibly also the entire granulator and in some cases also other subassemblies of the apparatus. Such operations for replacing the wearing parts of the granulator or the complete granulator are extremely time-intensive and cause relatively high costs, above all because at least one complete granulator per apparatus must be held in stock in many cases, in particular in cases where it is impossible to replace individual components of the granulator for reasons of time due to the type of application of the apparatus.
In this connection it must in particular also be considered that one main area of application for granulators of the abovementioned type is in continuous operation, whereby for reasons of process technology continuous plants cannot be switched off, i.e. in such cases the granulating process must run on practically without interruption.
There are also cases of application with discontinuous operation, but here too there must be practically no interruption during discharge, i.e. during the granulation time, since this would clearly worsen the quality of the resulting product.
The granulators of known apparatuses have the further disadvantage that if the draw-in rolls are blocked by the product the places to be cleaned are difficult to reach.
During operation of known apparatuses the plastic strands, after entering the granulator, are drawn by the pair of draw-in rolls disposed therein, whereby it must be considered that there is a relatively great distance between the nozzles from which the strands emerge in molten form and this pair of draw-in rolls of the granulator. The consequence is that initially molten plastic strands are drawn by soon hardened plastic strands in uncontrolled fashion over a relatively great distance. Depending on how the types of plastic are used, this might cause twisting or adherence of the plastic strands so that the resulting product, i.e. the granules, are irregular.